1. Field of the Invention
This invention relates to an electrophotographic image forming apparatus, and more particularly to a textured photoreceptor (i.e., imaging member) with very fine parallel grooves formed laterally upon the overcoat layer of the imaging member to reduce cleaning blade wear by reducing the adhesion between the cleaning blade and the overcoat layer of the imaging member without degrading image reproduction quality.
2. Description of Related Art
In electrophotography, an Imaging member, either belt or drum, containing an insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The insulating layer is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in certain areas of the insulating layer while leaving behind an electrostatic latent image in the other areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles (i.e., toner) on the surface of the insulating layer. The resulting visible image may then be transferred from the imaging member to a support, such as paper. This imaging process may be repeated many times with reusable insulating layers. It is necessary to clean residual toner from the surface of the insulating layer prior to repeating another imaging cycle, however.
One common method of cleaning is blade cleaning. Elastomeric blade cleaning of imaging members is conceptually simple and economical, but raises reliability concerns in mid and high volume applications due to apparent random failures. Such random failures justify the reluctance to include blade cleaners in higher volume machines with or without some backup cleaning element.
A number of methods have been proposed to enhance blade/imaging member contact properties. One method includes agitation of the blade against the imaging member to prevent build-up of material along the contact seal. Another method includes addition of redundant members, such as disturber brushes to loosen or collect debris which might otherwise stress the blade element. These methods increase the mechanical complexity and the cost of the cleaning assembly, and are thus undesirable.
Another method for enhancing blade/imaging member contact properties includes the addition of lubricants to the toner, imaging member and/or blade. However, this method increases the material complexity and introduces compatibility problems. This often results in films developing on the imaging member which hinder imaging member function and degrade image quality.
A further proposal for enhancing blade/imaging member contact properties is by roughening the imaging member surface to reduce the blade friction and the blade/imaging member contact area. This method may also introduce compatibility problems depending on how the roughened surface is introduced. For example, particulate additives to the bulk of the transport layer to provide roughness through surface asperities can degrade electrical and/or mechanical properties. Surface asperities can be worn away in normal machine copying, limiting any cleaning benefit. Surface roughening can also have direct adverse effects such as the introduction of sites against which toner may become lodged. Imaging member surface roughening can also inhibit cleaning by allowing the blade to pass over toner and other surface debris.
One of the most common "predictable" or non-random blade cleaning failures is permanent impaction of toner particles and toner fragments. This type of failure is generally encountered and resolved during program development. It involves material, including toner particles, which becomes impacted onto the imaging surface and adheres with such force that the material cannot be removed by the cleaning elements. Additionally debris, including untransferred toner residue and developer and/or toner additives, may become jammed against an asperity on the imaging member surface. Repeated passes and extended copying can lead to the build-up of an elongated crusty deposit in front of the asperity which eventually print out as spots on the copy.
Various strategies have been implemented or proposed to deal with this type of blade cleaning problem, including those enumerated above. Additional approaches to the resolution of such problems include the elimination of the material which impacts or builds-up in the tail, the inclusion of additives which lubricate and/or scavenge the offending material, and the development of an imaging surface which resists toner impaction and/or build-up.
Optimally, all residual toner is being totally removed by the cleaning blade with each rotation of the imaging member drum or belt surface. The entire surface must be thoroughly cleaned thousands of times without damage. The cleaning loads on the blade are necessarily uneven, both short term and long term, because the location, density and tenacity of the residual toner varies widely over the surface, depending on the images, exposures, surface charge, toner development and image location. The required frictional/adhesional forces for effective blade cleaning have been high, particularly for the desired combination of a relatively soft elastomeric blade cleaning tip edge tightly engaging an imaging member surface, which imaging member surface must be smooth enough to provide high optical resolution images. Unless carefully controlled, these adhesional forces can also result in the generation of excessive pressure or heat, resulting in physical and chemical changes in the toner, smearing of toner materials onto the imaging member or blade, excessive imaging member or blade wear, or other problems, especially in higher speed machines. Thus, cleaning dry toner from an imaging member presents extremely critical requirements not normally found in other cleaning fields, and blade cleaning systems suitable for other fields and applications, e.g., cleaning or doctoring systems for metal gravure rollers, inking rollers, paper mill rollers or adhesive applicators, are not normally appropriate.
For electrophotographic purposes, an imaging member having an overcoat layer formed c,n the photosensitive layer is known. The purpose of the overcoat layer is for improving the durability and anti-abrasion properties of the photosensitive layer surface. For instance, U.S. Pat. No. 4,764,448 to Yoshitomi et al., disclose an amorphous silicon imaging member having a specific surface roughness obtained by polishing the surface using soft abrasive substances. The polished surface prevents image blurring in the imaging member.
U.S. Pat. No. 4,904,557 to Kubo, discloses an imaging member comprising a photosensitive layer having a surface roughness of ten points over a reference length of 2.5 mm The particular surface roughness is provided to prevent an interference fringe pattern appearing at image formation, and for preventing black dots from appearing at reversal development.
U.S. Pat. No. 4,537,849 to Arai, discloses a photosensitive element having a roughened selenium-arsenic alloy surface. The outer conductive surface is roughened by direct mechanical grinding (polishing). A roughness of less than or equal to 3 micrometers laterally and from 0.1 to 2 micrometers in height is disclosed for reducing adhesion of transfer paper or toner.
U.S. Pat. Nos. 3,992,091 and 4,076,564 to Fisher, disclose roughened imaging surfaces of a xerographic imaging member. Roughening of the imaging member surface is achieved indirectly by first chemically etching a substrate. The substrate is then uniformly coated with photoconductive material which conforms to the surface in such a way that the substrate roughness is reproduced on a photoconductive surface. The level of roughness may be from 3 to 5 or 10 to 20 micrometers laterally with a 1 to 2 micrometers height.
U.S. Pat. No. 4,134,763 to Fujimura et al., discloses a method for making the surface of a substrate rougher by bringing a grinding stone in light pressure contact with the surface of the substrate, such that small vibrations form a minute roughness on the surface of the substrate. The substrate surface roughness is preferably from 0.3 micrometers to 2 micrometers. The rough surface of the substrate improves adhesion between the substrate and a selenium layer. Unlike the Fisher patents, the roughness of the substrate is not disclosed as being reproduced in the photosensitive layer.
U.S. Pat. No. 4,804,607 to Atsumi, discloses an overcoat layer which is a film-shaped inorganic material overcoating the surface of the photosensitive layer. The overcoat layer is formed such that a rough surface is provided having 500 to 3000 convexities and concavities per 1 centimeter linear distance with a maximum depth difference of .05 to 1.5 micrometers between the convexities and the concavities. The convexities and concavities are formed by heating the support, photosensitive layer and the overcoat layer.
U.S. Pat. No. 4,693,951 to Takasu et al., disclose an image bearing member having a maximum (vertical) surface roughness of 20 micrometers or less, and an average surface roughness which is less than or equal to two times a toner particle size.
U.S. Pat. No. 5,187,039 to Meyer, discloses using an imaging member having a surface roughness which prevents the adhesion of toner particles, especially flat toner particles, during blade cleaning. The surface roughness is defined by: ##EQU1## where R is an average height of asperities of the surface, a.sub.nn is one-half the nearest neighbor distance between the asperities on the surface, K.sub.B is bulk modulus of the blade, .sigma. is Poisson's ratio of the toner composition, E is Young's modulus of tile toner composition, t is an average thickness of flat particles in the toner composition, a.sub.f is an average radius of the flat particles, .mu. is an average of toner-blade and toner-surface friction coefficients, .GAMMA. is the Dupre work of adhesion between the surface and the flat particles, and .crclbar. is blade tip angle. The particular surface roughness prevents toner particles from developing a high surface energy on the imaging surface.
While the above described imaging members provide a roughened surface for various purposes, the references do not teach or suggest a particular surface texture which would be desirable for preventing the adhesion of toner particles, and in particular, to reduce adhesion between the cleaning blade and the imaging member, thereby extending the cleaning blade's useful life.